S-parameters measurement for very low VNA source power

Hello everybody,

First post on this forum. I want to measure the S-parameters of a DUT. My problem is that to avoid compression of the DUT, I have to set the source powers of the VNA at levels below -60dBm. By doing so, internal attenuators of VNA are commutated, making the calibration impossible...
If I make the calibration at decent source power level (0dBm) and then decrease the source power to level below -60dBm, then the S paramater displayed are completely inconsistent.
Any idea to fix my problem?

Thanks

Issam

Calibrate at -60dBm instead 0dBm.

If your VNA source power too low , the Signal to noise ratio is no enough.
The signal will near the noise level.

How many power will be output from your DUT?

VNA damage power level normally is +27dBm.
VNA maximum input level normally is +15dBm.
Above +15dBm , the VNA receiver will be compression.

I suggest you set the VNA power level at -20dBm or -30dBm.
Depend on your DUT output power.

If your DUT output power is below 15dBm , use the full two port TOSM method ( Open Short , load 50 Ohm , through cal kit ) to calibrate the VNA.

If your DUT output power is above 15dBm , you can add the attenuator at the VNA test cable input port.
Use the normalization method ( through cal kit) to calibrate the cable and attenautor loss.
This is useful for gain measurement.

Hi nccu1990, Kaspalla,

Thanks for your answer. Actually I want only to focus on S11 parameters.

Kaspalla,
I cannot calibrate the VNA at power levels below -60dBm, the VNA returns an error mentionning that it cannot calculate the error cofficient for the reference load (open, short..). I assume this is due to the attenuation commutated inside the VNA for these levels.

nccu1990,

I will definitively saturate my DUT for power level around -20dBm, which can skew the S11 parameter measurement. Moreover, putting an attenuator between VNA input port and DUT will false the S11 calibration.

Any other suggestions.

regards,

Issam

You may want to consult your VNA manual to understand the relation of signal level and achievable S- pararameter measurement accuracy. Even if a VNA supports calibration at -60 dBm, the results must be expected to have a high uncertainty.

Why don't you start with the lowest source power suppported for calibration?

You can evaluate the reproducability of S parameter measurements for different source power levels first.

Then you can make a D.U.T measurement with different levels to identify the maximum level for a measurement unaffected by compression.

Hi Issamy,

I think that your DUT is low output power.
So don't need add the attenuator at the VNA test cable.

Use -25dBm or -30dBm power source , keep the power at the VNA dynamic range.

Some AN about VNA uncertainly , maybe useful for you.

VNA dynamic range and linearity accuracy AN

Which device are you testing?
If you set your VNA source power at least 10dB below its P1dB compression you should still get an accurate measure of its linear S11 performance at the lower power that you are interested in.